Hitherto, various curable liquid compositions which are cured to give rubbery materials have been developed. Among them, a curing system having good depth curability has been developed, which system is prepared by cross-linking a polyorganosiloxane having on the average two or more vinyl groups at a molecular end or in a molecular chain of one molecule, with a polyorganohydrogensiloxane having two or more hydrogen atoms bonded to silicon atoms in one molecule. Such a system is used as a sealing agent or a potting agent by utilizing its good weather resistance, water resistance and thermal resistance. However, such systems have limited applications because the cost is high, the adhesion property is not good and fungus easily grows. Further, said polyorganosiloxane generally has poor comparability with an organic polymer. When a polyorganohydrogensiloxane and an organic polymer having an alkenyl group are intended to be cured, hydrolysis and dehydrogenation condensation reaction of the polyorganohydrogensiloxane increase due to phase separation, and sufficient mechanical properties are not achieved because of voids.